Wireless communications systems use multiple access protocols to enable wireless communications between base transceiver stations and multiple subscriber units. Typically, a wireless communications system includes multiple base transceiver stations that are spaced apart to create subscriber cells. Subscriber units within the subscriber cells exchange information between nearby base transceiver stations over dedicated radio frequencies.
The use of wireless communications systems is rapidly expanding beyond the exchange of voice communications to include the exchange of broadband data, such as multimedia data. For example, a single wireless link between a base transceiver station and a subscriber unit may be utilized to simultaneously exchange voice, video, and data. In order to effectively deliver broadband services over a wireless link, the wireless link must be able to communicate at a higher rate than traditional voice-only wireless links. One technique that has been utilized to increase the communications rate of wireless links involves increasing the RF bandwidth that is used to transmit information between a base transceiver station and a subscriber unit. For example, where a traditional voice-only wireless link utilizes 200 kHz of RF bandwidth, a broadband wireless link may utilize 6 MHz of RF bandwidth. Although larger RF bandwidth enables an increased communications rate over a wireless link, the larger RF bandwidth also leads to larger thermal noise at the RF receiver. As is known in the field of RF communications, the thermal noise, N, at a receiver is expressed as:N=K·T·Bwhere: K=Boltzman constant                T=ambient temperature in degrees Kelvin, and        B=the RF bandwidth of the communications channelFrom the above expression, it can be seen that the thermal noise, N, is directly proportional to the RF bandwidth of the communications channel and is expressed as:N∝B        
Because wireless frequency bandwidth is a limited resource, wireless cellular systems often reuse the same wireless frequencies in different subscriber cells. Although the reuse of wireless frequencies in different subscriber cells frees up available bandwidth, the reuse of wireless frequencies also causes co-channel interference when signals from one subscriber cell are detected within another subscriber cell. The effects of co-channel interference can be controlled by ensuring that the same frequency is only reused in cells that are spaced apart by a sufficient distance. The magnitude of co-channel interference in subscriber cells is reduced by attenuation as electromagnetic waves from interfering channels travel the distance between co-channel cells. The way in which frequencies are reused in a wireless communications system (also referred to as the frequency reuse structure) depends on many factors including the number of subscriber cells that are in a particular cluster of cells. The co-channel interference (often measured as the signal to interference ratio, SIR) at a particular receiver is directly effected by the frequency reuse structure.
One important operating characteristic of receivers in wireless communications systems is the signal to interference and noise ratio (SINR). The SINR is defined as:SINR=C/(I+N)
where: C is the received channel signal,                I is the co-channel interference, and        N is the thermal noise.        
As stated above, the co-channel interference, I, at a receiver is directly effected by the frequency reuse structure of a wireless communications system and the thermal noise, N, at a receiver is directly proportional to the RF bandwidth of a wireless link. The signal strength, C, of a received signal depends on the distance between the transmitter and the receiver for a given transmitter power. As the distance between a transmitter and a receiver increases, the value of C decreases, which in turn results in a reduction of the SINR at the receiver. Because the channel to interference ratio (C/I) is relatively unaffected by the distance between a transmitter and a receiver, the reduction in SINR is primarily caused by a reduction in the signal to noise ratio (SNR) of a received signal, where SNR=C/N.
For successful operation of receivers in a wireless communications system, the value of SINR should be above a minimum threshold. Typically, the minimum threshold of SINR for a wireless link is a design parameter that is fixed by one or more of the required bit error rate (BER), the required packet error rate (PER), the coding rate, the modulation rate, and the channel conditions. Given the minimum SINR threshold, the effective size of a subscriber cell in a wireless communications system is set by the maximum distance between a transmitter and a receiver that can be achieved while maintaining the SINR above the minimum threshold. Because the C/I ratio is relatively unaffected by the distance between a transmitter and a receiver, the effective size of a subscriber cell is a function of the SNR of a received signal.
As stated above, the SINR is a design parameter that is fixed by one or more of the required bit error rate (BER), the required packet error rate (PER), the coding rate, the modulation rate, and the channel conditions. Further, the channel signal strength, C, is typically set by the transmitter characteristics. With regard to communications in the downlink direction, from a base transceiver station to a subscriber unit, the channel signal strength, C, can be increased as necessary, within regulatory limits, to maintain a given SINR in view of increased thermal noise that results from a larger RF bandwidth. However, with regard to communications in the uplink direction, from a subscriber unit to a base transceiver station, the channel signal strength, C, is limited by many factors and cannot be easily increased to compensate for an increase in thermal noise. For example, the channel signal strength of uplink transmissions is limited by power consumption concerns at the subscriber unit (particularly in mobile subscriber units), the cost of high power components (such as power amplifiers and filters), government regulations, and human health concerns.
Because of the limitations in uplink transmission power, the effective size of a wireless subscriber cell is often uplink limited. That is, as the distance between a subscriber unit and a base transceiver station increases, the channel signal strength decreases until finally the SINR drops below the minimum threshold. If a subscriber unit exceeds its uplink limit (also referred to as the noise limit) such that the SINR drops below the minimum threshold, the quality of service experienced at the subscriber unit drops accordingly. A reliable quality of service for each subscriber unit is a critical characteristic of a wireless communications system.
In addition to effecting the quality of service delivered to subscriber units in a wireless communications system, the uplink limit of a wireless link also defines the maximum size of a wireless cell. The maximum size of the wireless cell in turn effects the capital costs of building a wireless communications system.
In view of the relationship of the RF bandwidth to receiver noise and the limitations on uplink transmission strength, what is needed is a broadband wireless communications system that can adapt to deliver a constant quality of service to a subscriber unit.